An exemplary fuel cell shutdown process is set forth in U.S. Pat. No. 7,141,324. The process seeks to avoid the presence of oxygen during shutdown. The desired result is to extend the time that the stack can maintain a fuel-rich environment (such as greater than a 2:1 fuel to air stoichiometry).
The process disconnects the main load, connects an auxiliary load or shorting resistor, blocks the entry of oxygen from the source, such as air, while flowing hydrogen into the anodes with fuel recycling, and while recycling cathode exhaust to the cathode inlets.
When the cell voltage drops to 0.2 volts per cell, the process in the aforementioned patent assumes that substantially all of the oxygen within the cathode flow field, and any oxygen that has diffused across the electrolyte to the anode flow field will have been consumed. Once the voltage per cell reaches 0.2 volts, both the anode and the cathode receive fresh hydrogen, with both the anode and cathode recycle operative.
This has the effect of not only reducing the amount of oxygen in the cathodes by electrochemical reaction but also eventually stabilizing the cathodes and the anodes with approximately the same partial pressure of hydrogen, along with inerts, mostly nitrogen, and traces of other gases, as the oxygen is consumed.
In the aforesaid patent, the process is continued until the cathodes and anodes have hydrogen concentrations greater than 90% as determined by hydrogen concentration sensors for the anodes and cathodes. Other processes may measure the oxygen and continue the process until the oxygen is a fractional percent.
Current methods rely heavily on optimization of the shutdown procedure by tailoring it until the desired results are obtained. Essentially, such processes all attempt to consume all the oxygen in the air-side flow fields, manifolds and associated plumbing and leave sufficient hydrogen in the air-side to consume any residual oxygen and oxygen drawn in through leaks.
One factor is the precision at which the air-side hydrogen level is set at the end of the shutdown procedure. Too little hydrogen will shorten the time of a fuel-rich stoichiometry, and too much hydrogen will cause excessive emissions if the power plant is restarted soon after shutdown.
Currently available hydrogen sensors are not reliable and do not accurately represent hydrogen concentrations in all of the anodes and in all of the cathodes. The position of hydrogen sensors pose additional reliability problems.
Another problem with these processes is that they can consume more time than is desirable in automotive applications. Shutdown procedure times of greater than seconds are undesirable in vehicles.